Skin is one of the largest organs in the body and covers substantially the entire outer surface of the body. Skin is composed of two main layers: the surface epithelium or epidermis, which contains keratinocytes as one type of epidermal cells, and the subjacent connective tissue layer or dermis, which contains fibroblasts as one type of dermal cells. The functions of skin include protecting an organism from injury and dessication by serving as a barrier to infection, perceiving or detecting environmental stimuli, excreting various substances, regulating body temperature, and helping to maintain water balance. Because of its quantitative and qualitative importance, substantially intact and healthy skin is important, not only for the well being of an organism but for its very survival.
The health and integrity of skin may be compromised by congenital or acquired pathologic conditions, either acute or chronic, for which normal skin regeneration and repair processes may be inadequate. These conditions include burns, wounds, ulcers, infections, diseases and/or congenital abnormalities. Patients who are burned over a large surface area often require immediate and extensive skin replacement. Less life-threatening but chronic skin conditions, as occur in venous stasis, diabetic or decubitus ulcers as three examples, may progress to more severe conditions if left untreated, particularly because patients with these conditions have an underlying pathology. Reducing the morbidity and mortality in such patients depends upon timely and effective restoration of the structure and function of skin.
Skin substitutes derived either ex vivo or in vitro may be used to treat these or other conditions. Desirable properties of skin substitutes are ready availability, a minimum requirement for donor skin, relative simplicity to produce, and cost-effectiveness of fabrication and use. Several approaches to fabrication of skin substitutes which satisfy some or all of these requirements have been attempted, with varying degrees of success. However, no skin substitute has yet regenerated all of the structures and functions of skin. Rather, all are subsets of uninjured skin. Only a transplant of full thickness skin restores virtually all the structures and functions of normal uninjured skin, but furthermore, scars during healing.
Materials have been manufactured for therapeutic use in skin repair. These materials contain different components replacing or substituting the structures and functions of the dermis and/or epidermis. Examples of these materials include EpiCel™, which lacks a dermal component and uses the patient's own cultured keratinocytes; Integra™, which uses a collagen-glycosaminoglycan (GAG) matrix to provide an acellular dermal component and uses a thin autograft; AlloDerm™ and a thin autograft; DermaGraft™, which uses a polyglycolic acid/polylactic acid (PGA/PLA) matrix and allogeneic human fibroblasts for the dermis; Hyaff/LaserSkin™, which uses hyaluran and fibroblasts for the dermis, and hyaluran and the patient's own keratinocytes for the epidermis; and PolyActive™, which uses polyethylene oxide/polybutylthalate (PEO/PBT) and may use the patient's own fibroblasts for the dermis, and the patient's cultured keratinocytes for the epidermis.
Materials to either temporarily cover wounds, or to stimulate permanent skin repair processes, include ApliGraft™, which uses collagen gel and allogeneic fibroblasts for the dermis, and cultured allogeneic keratinocytes for the epidermis; Comp Cult Skin™ or Orcel™, which uses collagen and allogeneic fibroblasts for the dermis, and cultured allogeneic keratinocytes for the epidermis; and TransCyte™ fibroblasts for the dermis and a synthetic material, BioBrane™, for the epidermis.
While the above materials are useful to varying degrees, each has disadvantages and limitations. Some of the materials are fragile mechanically, making it difficult to perform the required manipulations and transfers of the material in large sections without tearing. Instead, the materials must be used as smaller pieces, which makes coverage of large surface areas technically laborious for the physician and cosmetically undesirable for the patient due to scarring where grafts adjoin. The materials are also susceptible to microbial contamination, which is unacceptable for patients who are already at an increased risk for infection due to their compromised conditions. The materials show varying rates of engraftment and times to heal, both of which must be considered in selecting the advantages of a particular material over another for a particular patient. For example, a material which is otherwise acceptable but which takes longer to engraft and heal is less desirable, because a successful recovery includes as rapid a return to a normal routine as possible.
The inventor's own previous composite skin replacement, disclosed in U.S. Pat. No. 5,976,878, which is expressly incorporated by reference herein in its entirety, had been successfully used for therapeutic treatment of skin wounds. It was applied surgically in a single procedure, and contained a layer of cultured epidermal cells, an acellular polymeric dermal membrane component, and a substantially nonporous lamination layer on one surface of the dermal membrane component. The dermal membrane component was formed from collagen, or collagen and a mucopolysaccharide compound, and was laminated with the same collagen-, or collagen and mucopolysaccharide-containing solution with a volatile cryoprotectant. The substantially nonporous lamination layer may be located between the dermal component and the layer of cultured epidermal cells, promoting localization of epidermal cells on the surface of the dermal component and movement of nutrients to the cells of the cellular epidermal component. This composition can also be used to deliver biologically active molecules to the site where it is applied.
Desirable features of the above-described composite skin replacement included a more rapid rate of vascularization of the area covered by the material, decreased microbial contamination, increased nutrient supply, and improved epidermal barrier function, compared to other materials. Areas covered with the composite skin replacement required less time to engraft and heal, and the material was less susceptible to microbial contamination than reported for other materials. Other desirable features are that this material was relatively non-fragile and easy to handle, and could be generated relatively rapidly, for example, within the time frame in which a burn patient requires skin grafts. However, while no other alternative material has healed excised, full-thickness wounds more rapidly, and with as low an incidence of microbial contamination, limitations still exist. Thus, there remains a need to more closely approach structural and functional properties of normal uninjured skin.